AppModule.ts

import { NgModule, APP_INITIALIZER } from '@angular/core';
import { HttpClientModule, HttpClient } from "@angular/common/http";
import 'rxjs/add/operator/toPromise';

export function initializeApplication(httpClient: HttpClient): () => Promise<any> {
  return (): Promise<any> => {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        resolve();
      }, 3000);
    });
  };
}

@NgModule({
  imports: [HttpClientModule],
  providers: [
    { provide: APP_INITIALIZER, useFactory: initializeApplication, deps: [HttpClient] multi: true }
  ]
})
export class AppModule { }